The invention, in some embodiments, relates to the field of water treatment, and more particularly, but not exclusively, to methods and devices for treatment of carbon-containing aqueous waste.
Wastewater (i.e., carbon-containing aqueous waste) is water that contains contaminants including organic contaminants.
The amount of organic contaminants in wastewater is often expressed in terms of BOD or COD in units of mg/L. BOD (biological oxygen demand) is the mass of oxygen required for digestion of biodegradable contaminants in the wastewater by microorganisms. COS (chemical oxygen demand) is the mass of oxygen required for chemical oxidation of organic contaminants in the wastewater.
Total dissolved solids (TDS) in mg/L refers to minerals, salts, metals, cations, anions and small amounts of organic matter dissolved in the wastewater.
Total suspended solids (TSS) in mg/L refer to small suspended or colloidal particles that do not settle from the wastewater due to gravity alone.
In some cases, a measure of a specific type of contaminant, for example aromatic or metal content, in wastewater is also given.
Wastewater can be classified as untreated or raw (generally having a BOD>300 mg/L or a high chemical load) or as treated. Treated wastewater is wastewater that has been treated to have a certain organic contaminant level: Grade A: BOD<20 mg/L; Grade B: 20<BOD<150 mg/L; or Grade C: 150<BOD<300 mg/L.
Wastewater treatment is a process for removing contaminants from the wastewater to produce a liquid and a solid (sludge) phase, where the liquid phase in suitable for reuse or discharge, for example, being free of odors, suspended solids, and pathogenic bacteria
There are a number of typical stages of large-scale wastewater treatment.
In an initial stage (primary treatment), the wastewater is clarified: floating solids and hydrophobic materials are removed, e.g., by raking or skimming, respectively, together with or followed by settling of sludge.
In a following stage (secondary treatment), most of the organic contaminants in the liquid effluent from the initial stage are removed, typically by biological digestion for example using aerobic bacteria, to biologically oxidise the organic contaminants. The resulting product settles as a coagulated mass (floc). To increase the rate of digestion, the wastewater is often activated (aerated) during the digestion process.
In a terminal stage (tertiary treatment), all but a negligible portion of bacterial matter and remaining organic contaminants are removed from the liquid effluent from the secondary treatment, usually by filtration over sand or activated carbon, to produce a liquid effluent that is water of a quality suitable for discharge or reuse.
Wastewater with chemical contaminants (e.g., from industrial processes) are sometimes treated by other (additional) methods, tailored for a specific type of contaminant.
The sludge from the primary and secondary treatment is typically isolated in batches in holding tanks for digestion by aerobic or anaerobic bacteria during which the gaseous by-products (e.g., CH4) are collected for use as fuel. When digestion is complete, the remaining solids may be dried for use as an agricultural fertilizer component, incinerated, buried or disposed at sea.
It is known to use algae in wastewater treatment. Algae remove most contaminants associated with faeces and urine from sewage. Lipids useful as biofuels can be extracted from harvested algae.
The use of algae in wastewater treatment has been described, inter alia, in U.S. Pat. Nos. 2,658,310; 3,645,040; 3,780,471; 4,043,903; 4,209,388; 4,235,043; 4,253,271; 4,324,067; 4,341,038; 4,554,390; 5,910,254; 6,000,551; 6,156,561; 6,180,376; and 6,524,486.
Until closing down in May 2009, GreenFuel Technologies Corporation (Cambridge, Mass., USA) grew algae in a photosynthetic bioreactor fed with carbon dioxide emissions from a fossil fuel combustion source to produce fuels (methane, ethanol, biodiesel) and solids (for animal feed and bioplastics).
GreenShift Corporation (New York, N.Y., USA) has developed a bioreactor process that uses thermophilic cyanobacteria suspended in water to consume carbon dioxide emissions from a fossil fuel combustion source. The cyanobacteria consume the carbon dioxide and water, releasing oxygen and water vapour. The organisms also absorb nitrogen oxide and sulphur dioxide. Upon reaching maturity, the cyanobacteria settle to the bottom of the bioreactor for harvesting.
AquaFlow (Richmond, New Zealand), produces biofuel and fine chemicals from algae harvested from a municipal wastewater site in Blenheim, New Zealand. Harvested algae undergo a multistep process that includes concentration of algae into algal slurry, dewatering of the slurry to produce an algae paste which is refined.